1. Field of the Invention
The present invention relates to a monitor signal output circuit for outputting a monitor signal to monitor a voltage of a battery, a battery pack having a monitor signal output function, a battery voltage monitor circuit for monitoring a voltage of a battery in accordance with a monitor signal, a battery system and an apparatus each having a battery and a monitoring function for a voltage of the battery, a battery voltage monitor method for monitoring a voltage of a battery, and a battery voltage monitor program storage medium storing therein a program for a battery voltage monitor.
2. Description of the Related Art
Many of portable type of electronic apparatuses such as a note type of personal computer and the like are loaded with a battery for apparatuses.
In case of an apparatus which is operated on a desk, or in the event that a portable type of apparatus such as a note type of personal computer is operated on a desk, there is no need to consider such a matter that a supply of electric power for operating those types of apparatus operated by electric power from a commercial power supply through an AC adapter is interrupted. On the other hand, in case of an apparatus operative upon receipt of supply of electric power from a battery, there is a need for a user always to be aware of a residue of the battery. Particularly, in case of an information processing apparatus such as a notebook-sized personal computer and the like, there is a possibility that when the residue of the battery becomes zero (or the battery died), all of partially completed data will be erased. Accordingly, there is a need for a user to be aware of a state of consumption of the battery and to save the partially completed data onto a non-volatile storage medium such as a hard disk before the residue of the battery becomes zero.
For the purpose of preventing troubles involved in a situation that the residue of the battery becomes zero, the conventional notebook-sized personal computer and the like incorporates thereinto a system for monitoring the residue of the battery.
FIG. 11 shows a battery system having a battery residue monitor function.
A battery system 10 comprises a battery pack 20 and a battery voltage monitor section 30. The battery system 10 is incorporated into a notebook-sized personal computer 40. The battery pack 20 is connected with the battery voltage monitor section 30 in such a manner that a selection signal input terminal 20a of the battery pack 20, a battery voltage output terminal 20b of the battery pack 20, a power supply terminal 20c of the battery pack 20 and a ground terminal 20d of the battery pack 20 are connected with a selection signal output terminal 30a of the battery voltage monitor section 30, a battery voltage input terminal 30b of the battery voltage monitor section 30, a power supply terminal 30c of the battery voltage monitor section 30 and a ground terminal 30d of the battery voltage monitor section 30, respectively.
The battery voltage monitor section 30 shown in FIG. 11 comprises a DC-DC converter circuit 31 and a microcomputer 32 operative upon receipt of a supply of an electric power of 5.0 V from the DC-DC converter circuit 31.
The microcomputer 32 transmits a selection signal for optionally selecting any one; of three batteries S1, S2, and S3, which are connected in series within the battery pack 20, via the selection signal output terminal 30a of the battery voltage monitor section 30 and the selection signal input terminal 20a of the battery pack 20 to a battery protection circuit 21 of the battery pack 20. The battery protection circuit 21 transmits in accordance with the selection signal a monitor signal representative of a voltage of the battery selected by the selection signal via the battery voltage output terminal 20b of the battery pack 20 and the battery voltage input terminal 30b of the battery voltage monitor section 30 to the microcomputer 32. The microcomputer 32 converts the transmitted battery voltage into a digital signal to recognize the battery voltage in form of a digital value, so that the residue of the battery is recognized from the battery voltage.
The battery pack 20 shown in FIG. 11 is provided with two FETs (FET1 and FET2). The batteries S1, S2, and S3 are each a chargeable battery, for example, a lithium-ion battery. FET1 of the two FETs turns off, when a voltage of the batteries S1, S2, and S3 goes down to the lower limit, and thereby preventing an over discharge of the batteries S1, S2, and S3. FET2 turns off when the batteries S1, S2, and S3 are completely charged and further charged, and thereby preventing an over-charge of the batteries S1, S2, and S3.
FIG. 12 is a view showing an internal structure of the battery protection circuit of the battery pack 20 shown in FIG. 11. However, here, the prevention of the over discharge and the over charge of the batteries is not a main subject matter, and thus a circuit structure of the prevention of the over discharge and the over charge of the batteries and the associated description will be omitted.
The battery protection circuit 21 has: a selection signal input terminal 21a for inputting a selection signal; a monitor signal output terminal 21b for outputting a monitor signal representative of a voltage of a battery; a node connection terminal 21c connected to one end (a plus electrode of S1) of a plurality (three) of batteries S1, S2 and S3 connected in series; a node connection terminal 21d connected to a connecting point of two batteries S1 and S2; a node connection terminal 21e connected to a connecting point of two batteries S2 and S3; and a node connection terminal 21f connected to another, end (a minus electrode of S3) of the batteries S1, S2 and S3 connected in series. Both ends of the plurality (three) of batteries connected in series and the connecting points of the battery-to-battery are referred to as a xe2x80x98nodexe2x80x99.
The battery protection circuit 21 comprises three differential amplifiers AMP1, AMP2, and AMP3, and a multiplexer MPX. The three differential amplifiers AMP1, AMP2, and AMP3 are for inputting two nodes 21c and 21d, two nodes 21d and 21e, and two nodes 21e and 21f, respectively. Outputs of the three differential amplifiers AMP1, AMP2, and AMP3 are fed to the multiplexer MPX. The multiplexer MPX selects one of the three inputs in accordance with the selection signal entered through the selection signal input terminal 21a and outputs the selected one through the monitor signal output terminal 21b. 
The microcomputer 32 shown in FIG. 11 sequentially outputs the selection signal to optionally select the batteries S1, S2 and S3, and receives through an AD conversion the monitor signal representative of the associated voltages of the batteries S1, S2 and S3, and thus it is possible to know the voltages of the batteries S1, S2 and S3.
FIG. 13 is a graph showing a relation between a discharge time (a horizontal axis) and a battery voltage (a vertical axis) of a lithium-ion battery. FIG. 14 is a graph showing a relation between a battery voltage (a vertical axis) and a battery residue (a horizontal axis) of a lithium-ion battery.
As shown in FIG. 13, when a battery is used, a voltage of the battery is gradually lowered. When the voltage of the battery is monitored, as shown in FIG. 14, it is possible to know the residue of the battery from the voltage of the battery.
According to the microcomputer 32, it is possible to take measures to meet the situation that the battery is not usable, in such a manner that the microcomputer 32 detects the respective voltages of the batteries S1, S2 and S3 to know the residue of the batteries, so that the microcomputer 32 informs of the residue a user of the notebook-sized personal computer 40, and when the residue of the batteries draws to the limit in use, the microcomputer 32 alarms the user, and/or data are automatically saved.
Here, there will be explained an offset voltage of the differential amplifiers AMP1, AMP2, and AMP3 shown in FIG. 12 and a measurement error caused by the offset voltage.
FIG. 15 is a circuit diagram showing one (here, the differential amplifier AMP1) of the three differential amplifiers AMP1, AMP2 and AMP3 shown in FIG. 12.
The differential amplifier AMP1 comprises a differential amplifier AMP11 and resistances connected around the differential amplifier. AMP11. In FIG. 15, R1 and R2, which are applied to the resistances, imply resistance values as well as the resistances. xcex1 denotes an offset voltage of the differential amplifier AMP11.
When a voltage to be applied to a noninverting input of the differential amplifier AMP1 and a voltage to be applied to an inverting input of the differential amplifier AMP1 are expressed by V+ and Vxe2x88x92, respectively, an output voltage V0 of the differential amplifier AMP1 is given by equation (1) as set forth below.
V0=(R2/R1)(V+xe2x88x92Vxe2x88x92)+( (R1+R2)/R1)xc3x97xcex1xe2x80x83xe2x80x83(1)
FIG. 16 is a circuit diagram which is modified in the point that the inverting input of the differential amplifier shown in FIG. 15 is grounded. In this case, the an output voltage V0 of the differential amplifier AMP1 is given by equation (2) as set forth below.
V0=(R2/R1)xc3x97V++((R1+R2)/R1)xc3x97xcex1xe2x80x83xe2x80x83(2)
FIG. 17 is a circuit diagram which is modified in the point that in the preceding stage of the differential amplifier shown in FIG. 15, there are provided differential amplifier AMP12 and differential amplifier AMP13 which serve as a buffer.
The differential amplifier AMP12 and differential amplifier AMP13 are buffer circuits for noninverting input and inverting input of the differential amplifier AMP11 respectively. The differential amplifiers AMP12 and AMP13 serve to prevent a leakage of a current from the batteries S1xcx9cS3 by the resistances R1 and R2 for determining amplification factor of the differential amplifier AMP11. The offset voltage xcex1 by the differential amplifiers AMP12 and AMP13 is a common mode input of the differential amplifier AMP11 and is cancelled by the differential amplifier AMP11. Therefore, it is sufficient to consider only the offset voltage of the differential amplifier AMP11 also in the differential amplifier shown in FIG. 17. Thus, an output voltage V0 of the differential amplifier AMP1 is given, in a similar fashion to that of FIG. 15, by equation (1) as set forth below.
V0=(R2/R1)(V+xe2x88x92Vxe2x88x92)+((R1+R2)/R1)xc3x97xcex1xe2x80x83xe2x80x83(1)
As seen from equations (1) and (2), the amplification degree of the differential amplifier AMP1 is determined by resistance values of the resistances R1 and R2, and accuracy of the amplification depends on the accuracy of the resistance values. As seen from equations (1) and (2), however, accuracy of the amplification is determined by ratio R2/R1 of the resistance values of the two resistances R1 and R2, but not the absolute value of the resistance values of the resistances R1 and R2. consequently, it is possible to implement a high accuracy of amplification degree by making inside an LSI the resistances R1 and R2 which determine the amplification degree. That is, in the event that the resistances are made in the LSI, the absolute value of the resistances is involved in the error of xc2x120-30% due to a diffusion dispersion of an impurity. To the contrary, the ratio of the resistance values of the two resistances can be controlled with great accuracy, and thus it is possible to prevent a dispersion of the ratio from rising more than 0.05%.
To the contrary, with respect to the offset voltage xcex1 of the differential amplifier, a dispersion in the manufacturing process of an LSI is directly reflected. Thus, when a plurality of differential amplifiers are made within the same LSI, the offset voltages xcex1 of the plurality of differential amplifiers, which are made within the same LSI, are almost the same as one another, but are large in difference among chips.
For example, it is necessary for a notebook-sized personal computer to detect the battery residue in accuracy of 1% or more. However, in this case, for the offset voltage xcex1 of the differential amplifier, it brings about a result of 2-3% or more in dispersion when the offset voltage xcex1 is converted into the battery residue.
In order to obtain a great accuracy of output of a differential amplifier, it may be considered that a fine control circuit is added to regulate the offset voltage xcex1 to be zero. However, this involves problems of a working amount required for the regulation of the offset voltage and rising of a cost for preparing a circuit for the regulation. Further, the offset voltage xcex1 is associated with a temperature change and a secular change, and thus even if the offset voltage xcex1 is regulated in the manner as mentioned above, it is difficult to cope with changes of the offset voltage xcex1 due to the temperature change and the secular change.
In order to cope with the problems as mentioned above, Japanese Patent Laid Open Gazette Hei. 6-260851 proposes a scheme that both the noninverting input and the inverting input of the differential amplifier are grounded at the time of the offset measurement, and then an output voltage of the differential amplifier is measured, so that the offset voltage is detected including temperature change and the secular change.
However, the offset voltage xcex1 is concerned with both the plus case and the minus case; nevertheless the above-mentioned proposed scheme is permitted to measure only the plus case of the offset voltage xcex1. For this reason, according to Japanese Patent Laid Open Gazette Hei. 6-260851, there is proposed a scheme that the offset voltage is regulated beforehand so that the offset voltage always offers the plus even if the temperature change and the secular change are concerned.
However, also in case of the proposed scheme, there is a need to prepare a circuit for regulation of the offset voltage and to perform a working of regulation of the offset voltage. Thus, it is unavoidable that the cost rises.
In view of the foregoing, it is an object of the present invention to provide a monitor signal output circuit, a battery pack, a battery voltage monitor circuit, a battery system, an apparatus, a battery voltage monitor method, and a battery voltage monitor program storage medium, which need no pre-regulation, and are capable of measuring a battery voltage with great accuracy.
The present invention can be classified to two groups of a first group in which battery voltages are measured with great accuracy, and a second group in which battery voltages are measured with great accuracy, and in addition such a great accuracy of measurement is implemented by a circuit operative on a low voltage of the power source.
To achieve the above-mentioned objects, the present invention provides a first monitor signal output circuit, which belongs to the first group, comprising:
a differential amplifier for monitor signal output; and
a change-over circuit for changing over a connection between a plurality of batteries connected in series and an input of said differential amplifier in accordance with a selection signal so that said differential amplifier outputs a signal according to the selection signal of a plurality of signals including signals representative of voltages of the plurality of batteries and a voltage involved in a combination of predetermined two or more batteries.
To achieve the above-mentioned objects, the present invention provides a first battery pack, which belongs to the first group, incorporating a plurality of batteries connected in series, and a monitor signal output circuit having a differential amplifier which outputs a signal selected in accordance with a selection signal of a plurality of signals including signals representative of voltages of the plurality of batteries and a voltage involved in a combination of predetermined two or more batteries.
In the battery pack according to the present invention as mentioned above, it is preferable that said battery pack further incorporates a battery voltage monitor circuit having:
an offset computing section for computing an offset voltage of said differential amplifier in accordance with a plurality of signals derived from said monitor signal output circuit; and
an offset correction section for correcting measurement errors of voltages of the plurality of batteries using the offset voltage computed in said offset computing section.
In this case, it is acceptable that said offset computing section computes the offset voltage of said differential amplifier in accordance with three signals representative of a first voltage involved in at least one part of battery of the plurality of batteries, a second voltage involved in at least one another part of battery of the plurality of batteries, and a third voltage involved in two or more batteries in combination of said part of battery and said another part of battery.
To achieve the above-mentioned objects, the present invention provides a first battery voltage monitor circuit comprising:
an offset computing section for computing an offset voltage of a differential amplifier, which outputs a signal selected in accordance with a selection signal of a plurality of signals including signals representative of voltages of a plurality of batteries connected in series and a voltage involved in a combination of predetermined two or more batteries, in accordance with a plurality of signals derived from a monitor signal output circuit having said differential amplifier; and
an offset correction section for correcting measurement errors of voltages of the plurality of batteries using the offset voltage computed in said offset computing section.
In the battery voltage monitor circuit according to the present invention as mentioned above, it is acceptable that said offset computing section computes the offset voltage of said differential amplifier in accordance with three signals representative of a first voltage involved in at least one part of battery of the plurality of batteries, a second voltage involved in at least one another part of battery of the plurality of batteries, and a third voltage involved in two or more batteries in combination of said part of battery and said another part of battery.
To achieve the above-mentioned objects, the present invention provides a first battery system comprising:
a plurality of batteries connected in series;
a monitor signal output circuit having a differential amplifier which outputs a signal selected in accordance with a selection signal of a plurality of signals including signals representative of voltages of the plurality of batteries and a voltage involved in a combination of predetermined two nor more batteries; and
a battery voltage monitor circuit having an offset computing section for computing an offset voltage of said differential amplifier in accordance with a plurality of signals derived from said monitor signal output circuit, and an offset correction section for correcting measurement errors of voltages of the plurality of batteries using the offset voltage computed in said offset computing section.
To achieve the above-mentioned objects, the present invention provides a first apparatus operative upon receipt of supply of an electric power, said apparatus comprising:
a plurality of batteries connected in series;
a monitor signal output circuit having a differential amplifier which outputs a signal selected in accordance with a selection signal of a plurality of signals including signals representative of voltages of the plurality of batteries and a voltage involved in a combination of predetermined two or more batteries; and
a battery voltage monitor circuit having an offset computing section for computing an offset voltage of said differential amplifier in accordance with a plurality of signals derived from said monitor signal output circuit, and an offset correction section for correcting measurement errors of voltages of the plurality of batteries using the offset voltage computed in said offset computing section.
To achieve the above-mentioned objects, the present invention provides a first battery voltage monitor method comprising steps of:
computing an offset voltage of a differential amplifier, which outputs a signal selected in accordance with a selection signal of a plurality of signals including signals representative of voltages, of a plurality of batteries connected in series and a voltage involved in a combination of predetermined two or more batteries, in accordance with a plurality of signals derived from a monitor signal output circuit having said differential amplifier; and
correcting measurement errors of voltages of the plurality of batteries using the offset voltage computed in said offset computing section.
To achieve the above-mentioned objects, the present invention provides a first battery voltage monitor program storage medium storing a battery voltage monitor program which causes a computer to operate as an apparatus for monitoring a voltage of a battery when the battery voltage monitor program is executed in said computer,
wherein said battery voltage monitor program storage medium stores the battery voltage monitor program having an offset computing section for computing an offset voltage of a differential amplifier, which outputs a signal selected in accordance with a selection signal of a plurality of signals including signals representative of voltages of a plurality of batteries connected in series and a voltage involved in a combination of predetermined two or more batteries, in accordance with a plurality of signals derived from a monitor signal output circuit having said differential amplifier; and
an offset correction section for correcting measurement errors of voltages of the plurality of batteries using the offset voltage computed in said offset computing section.
According to the above-mentioned invention belonging to the first group, an offset voltage of the differential amplifier is measured utilizing such a feature that the offset voltage is constant in both cases that a voltage of any one of a plurality of batteries connected in series is measured, or that voltages of two or more batteries connected in series is measured, and a measurement error is corrected from the measured battery voltage. Thus, it is possible to measure and correct the offset voltage at the time of measurement, including changes of temperature and secular change, without provision of an adjustment in advance, and thereby measuring the battery voltage with great accuracy.
To achieve the above-mentioned objects, the present invention provides a second monitor signal output circuit, which belongs to the second group, comprising:
a differential amplifier for monitor signal output; and
a change-over circuit for selecting in accordance with a selection signal any one of a plurality of signal pairs consisting of a signal pair representative of voltages on two nodes, which are selected in accordance with the selection signal, of a plurality of nodes consisting of both ends of a plurality of voltage sources connected in series and connecting points of the voltage sources, and a signal pair representative of a voltage of one battery, which is selected in accordance with the selection signal, of a plurality of batteries connected in series and slice voltages on said plurality of voltage sources, said change-over circuit feeding a selected signal pair to said differential amplifier.
To achieve the above-mentioned objects, the present invention provides a second battery pack, which belongs to the second group, incorporating a plurality of batteries connected in series, and a monitor signal output circuit having a differential amplifier which outputs a signal selected in accordance with a selection signal of a plurality of signals consisting of a signal representative of a voltage between two nodes, which are selected in accordance with the selection signal, of a plurality of nodes consisting of both ends of a plurality of voltage sources connected in series and connecting points of the voltage sources, and a signal representative of a difference voltage between a voltage of one battery, which is selected from among the plurality of batteries in accordance with the selection signal and slice voltages on said plurality of voltage sources.
In the battery pack according to the present invention as mentioned above, it is acceptable that said battery pack further incorporates said plurality of voltage sources.
Alternatively, it is acceptable that said plurality of voltage sources are adapted for an external connection, and said battery pack has terminals to which nodes of said plurality of voltage sources are connected.
Further, it is also acceptable that said battery pack further incorporates a battery voltage monitor circuit having:
an offset computing section for computing an offset voltage of said differential amplifier in accordance with a plurality of signals derived from said monitor signal output circuit, each of said plurality of signals being representative of a voltage between nodes in which at least one node is different from another node; and
an offset correction section for correcting measurement errors of voltages of the plurality of batteries in accordance with the signals representative of difference voltages associated with the plurality of batteries, derived from said monitor signal output circuit, and the offset voltage computed in said offset computing section. In this case, it is acceptable that said offset computing section computes the offset voltage of said differential amplifier in accordance with three signals representative of a first voltage involved in a first voltage source of the plurality of voltage sources, a second voltage involved in a second voltage source of the plurality of voltage sources, and a third voltage involved in a voltage source in combination of said first voltage source and said second voltage source.
To achieve the above-mentioned objects, the present invention provides a second battery voltage monitor circuit, which belongs to the second group, comprising:
an offset computing section for computing an offset voltage of a differential amplifier, which outputs a signal selected in accordance with a selection signal of a plurality of signals consisting of a signal representative of a voltage between two nodes, which are selected in accordance with the selection signal, of a plurality of nodes consisting of both ends of a plurality of voltage sources connected in series and connecting points of the voltage sources, and a signal representative of a difference voltage between a voltage of one battery, which is selected from among the plurality of batteries in accordance with the selection signal and slice voltages on said plurality of voltage sources, in accordance with a plurality of signals derived from a monitor signal output circuit having said differential amplifier, each of said plurality of signals being representative of a voltage between nodes in which at least one node is different from another node; and
an offset correction section for correcting measurement errors of voltages of the plurality of batteries in accordance with the signals representative of difference voltages associated with the plurality of batteries, derived from said monitor signal output circuit, and the offset voltage computed in said offset computing section.
In the second battery voltage monitor circuit according to the present invention as mentioned above, it is acceptable that said offset computing section computes the offset voltage of said differential amplifier in accordance with three signals representative of a first voltage involved in a first voltage source of the plurality of voltage sources, a second voltage involved in a second voltage source of the plurality of voltage sources, and a third voltage involved in a voltage source in combination of said first voltage source and said second voltage source.
To achieve the above-mentioned objects, the present invention provides a second battery system, which belongs to the second group, comprising:
a plurality of batteries connected in series;
a plurality of voltage sources connected in series;
a monitor signal output circuit having a differential amplifier which outputs a signal selected in accordance with a selection signal of a plurality of signals consisting of a signal representative of a voltage between two nodes, which are selected in accordance with the selection signal, of a plurality of nodes consisting of both ends of said plurality of voltage sources connected in series and connecting points of the voltage sources, and a signal representative of a difference voltage between a voltage of one battery, which is selected from among the plurality of batteries in accordance with the selection signal and slice voltages on said plurality of voltage sources; and
a battery voltage monitor circuit having an offset computing section for computing an offset voltage of said differential amplifier in accordance with a plurality of signals derived from said monitor signal output circuit, each of said plurality of signals being representative of a voltage between nodes in which at least one node is different from another node, and an offset correction section for correcting measurement errors of voltages of the plurality of batteries in accordance with the signals representative of difference voltages associated with the plurality of batteries, derived from said monitor signal output circuit, and the offset voltage computed in said offset computing section.
To achieve the above-mentioned objects, the present invention provides a second apparatus operative upon receipt of supply of an electric power, which belongs to the second group, said apparatus comprising:
a plurality of batteries connected in series;
a plurality of voltage sources connected in series;
a monitor signal output circuit having a differential amplifier which outputs a signal selected in accordance with a selection signal of a plurality of signals consisting of a signal representative of a voltage between two nodes, which are selected in accordance with the selection signal, of a plurality of nodes consisting of both ends of said plurality of voltage sources connected in series and connecting points of the voltage sources, and a signal representative of a difference voltage between a voltage of one battery, which is selected from among the plurality of batteries in accordance with the selection signal and slice voltages on said plurality of voltage sources; and
a battery voltage monitor circuit having an offset computing section for computing an offset voltage of said differential amplifier in accordance with a plurality of signals derived from said monitor signal output circuit, each of said plurality of signals being representative of a voltage between nodes in which at least one node is different from another node, and an offset correction section for correcting measurement errors of voltages of the plurality of batteries in accordance with the signals representative of difference voltages associated with the plurality of batteries, derived from said monitor signal output circuit, and the offset voltage computed in said offset computing section.
To achieve the above-mentioned objects, the present invention provides a second battery voltage monitor method, which belongs to the second group, comprising steps of:
computing an offset voltage of a differential amplifier, which outputs a signal selected in accordance with a selection signal of a plurality of signals consisting of a signal representative of a voltage between two nodes, which are selected in accordance with the selection signal, of a plurality of nodes consisting of both ends of a plurality of voltage sources connected in series and connecting points of the voltage sources, and a signal representative of a difference voltage between a voltage of one battery, which is selected from among the plurality of batteries in accordance with the selection signal and slice voltages on said plurality of voltage sources, in accordance with a plurality of signals derived from a monitor signal output circuit having said differential amplifier, each of said plurality of signals being representative of a voltage between nodes in which at least one node is different from another node; and
correcting measurement errors of voltages of the plurality of batteries in accordance with the signals representative of difference voltages associated with the plurality of batteries, derived from said monitor signal output circuit, and the offset voltage computed in said offset computing section.
To achieve the above-mentioned objects, the present invention provides a second battery voltage monitor program storage medium storing a battery voltage monitor program which causes a computer to operate as an apparatus for monitoring a voltage of a battery when the battery voltage monitor program is executed in said computer,
wherein said battery voltage monitor program storage medium stores the battery voltage monitor program having an offset computing section for computing an offset voltage of a differential amplifier, which outputs a signal selected in accordance with a selection signal of a plurality of signals consisting of a signal representative of a voltage between two nodes, which are selected in accordance with the selection signal, of a plurality of nodes consisting of both ends of a plurality of voltage sources connected in series and connecting points of the voltage sources, and a signal representative of a difference voltage between a voltage of one battery, which is selected from among the plurality of batteries in accordance with the selection signal and slice voltages on said plurality of voltage sources, in accordance with a plurality of signals derived from a monitor signal output circuit having said differential amplifier, each of said plurality of signals being representative of a voltage between nodes in which at least one node is different from another node; and
an offset correction section for correcting measurement errors of voltages of the plurality of batteries in accordance with the signals representative of difference voltages associated with the plurality of batteries, derived from said monitor signal output circuit, and the offset voltage computed in said offset computing section.
According to the above-mentioned invention, which belongs to the second group, an offset voltage of the differential amplifier is determined through a measurement of voltages of a plurality of voltage sources connected in series, and slice voltages on the plurality of voltage sources are determined from the differential amplifier, so that the battery voltage subtracting the slice voltage is outputted to monitor the battery voltage. This feature makes it possible to monitor the battery voltage with greater accuracy by a circuit of a low voltage.
Particularly, in case of the lithium-ion battery having characteristics shown in FIGS. 13 and 14, the voltage on a state that it is completely charged is about 4V, and the voltage on a state that it is discharged up to the degree unavailable is 3V. Thus, the difference is 1V or so. Therefore, determination of a suitable slice voltage makes it possible to sufficiently monitor the battery voltage with a circuit operative on the battery voltage of, for example, 3V or so.